1. Field of the Invention
The present invention is directed to an improvement in the process for the carbonylation of methanol to produce acetic acid. More specifically, the improved method of the present invention reduces the formation of carbonyl impurities in the carbonylation reaction by way of conducting the reaction with low amounts of methyl iodide.
2. The Related Art
There are a number of currently-employed processes for producing acetic acid. One of the most useful commercially is the carbonylation of methanol with carbon monoxide, now known as the Monsanto process. This methanol carbonylation process, as exemplified by U.S. Pat. No. 3,769,329 ("the '329 patent") issued to Paulik and assigned to the Monsanto Company, is the process used to produce the majority of the acetic acid commercially worldwide.
The process utilizes a catalyst comprised of rhodium, either dissolved or otherwise dispersed in the liquid reaction medium and a halogen-containing catalyst promoter as exemplified by, preferably, methyl iodide. Rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The '329 patent discloses a very large number of suitable halide promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
The '329 patent indicates that the liquid reaction medium can be any solvent compatible with the catalyst system and that it may comprise, for example, the pure alcohol which is being reacted, or mixtures thereof with the desired carboxylic acid end product and/or esters of these two compounds. The preferred solvent and liquid reaction medium for the process is the desired carboxylic acid itself, i.e., acetic acid when methanol is being carbonylated to produce acetic acid. The reaction medium is preferably comprised of rhodium, methanol, methyl iodide, methyl acetate, acetic acid, and water.
Importantly, the '329 patent indicates that a substantial quantity of water should be present in the reaction mixture in order to attain a satisfactorily high reaction rate. Furthermore, the patent indicates that reducing the water content of the reaction medium leads to the production of ester product as opposed to carboxylic acid. Indeed, European Patent Application 055,618, also assigned to Monsanto Company, indicates that typically about 14-15 weight percent (wt % ) water is present in the reaction medium of a typical acetic acid plant using this technology. Likewise, Hjortkjaer and Jensen [Ind. Eng. Chem., Prod Res. Dev. 16, 281-285 (1977)] have shown that increasing the water from 0 to 14 wt % water increases the reaction rate of methanol carbonylation.
European Patent Application EP 055, 618 indicates that rhodium tends to precipitate out of the reaction medium. This tendency is most pronounced during the course of distillation operations to separate the product acetic acid from the reaction medium when the carbon is monoxide content of the catalyst system is reduced. The tendency for rhodium to precipitate out of the reaction medium increases as the water content of the reaction medium is decreased. Accordingly, based on the teachings of the '329 patent and European Patent Application EP 055,618, a substantial quantity of water is required in the reaction medium in order to combat the tendency for rhodium to precipitate, i.e., to maintain catalyst stability.
Preferably, commercial acetic acid is anhydrous or nearly anhydrous ("glacial"). Recovering acetic acid in anhydrous or nearly anhydrous form from a reaction medium comprising 14-15 wt % water, i.e., separating the acetic acid from the water, involves substantial expenditure of energy in distillation and/or additional processing steps.
Improvements have been made to the basic Monsanto process exemplified by the '329 patent. Of interest for the purposes of the present invention are those improvements which have allowed the operation of the process at water concentrations below 14 wt %. Commonly assigned U.S. Pat. Nos. 5,001,259; 5,026,908; 5,144,068; and European Patent No. 161,874B2 all provide improved methods of carbonylating methanol wherein the water content is maintained substantially below 14 wt %. As disclosed in those patents, acetic acid is produced from methanol in a reaction medium comprising methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically effective concentration. The patents also disclose the unexpected discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e., 4 wt % or less, in the reaction medium (despite the general industry practice of maintaining approximately 14-15 wt % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, and a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. These patents teach that the concentration of methyl acetate and iodide salts are significant parameters affecting the rate of carbonylation of methanol to produce acetic acid especially at low water concentrations.
In general, U.S. Pat. No. 5,144,068 and the related patents noted above teach that high levels of methyl iodide are desirable. Note FIGS. 4, 16 and 22 of the '068 patent, as well as Table 2, at column 9, lines 41-54 of the '068 patent.
By using relatively high concentrations of methyl iodide, methyl acetate and an iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the reaction medium contains extremely low water concentrations. Thus, these patented processes allow the production of acetic acid at lower water concentrations than previously known in the prior art. U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068 and European Patent No. 0 161 874 B2 are herein incorporated by reference.
Nonetheless, as the methanol carbonylation process has been practiced at increasingly lower water concentrations other problems have been found to have arisen. Specifically, operating at this new lower water regime has exacerbated certain impurities in the product acetic acid. As a result, the acetic acid product formed by the above-described low water carbonylation is frequently deficient with respect to the permanganate time owing to the presence therein of small proportions of residual impurities. Since a sufficient permanganate time is an important commercial test which the acid product must meet for many uses, the presence therein of such impurities that decrease permanganate time is objectionable [Ullman's Encyclopedia of Industrial Chemistry, "Acetic Acid", Volume A1, p 56, 5.sub.th ed]. Of particular concern are certain carbonyl compounds and unsaturated carbonyl compounds, particularly acetaldehyde and its derivatives, crotonaldehyde and 2-ethyl crotonaldehyde (also referred to as unsaturated impurities). However other carbonyl compounds known also to effect the permanganate time are acetone, methyl ethyl ketone, butyraldehyde, and 2-ethyl butyraldehyde. Thus, these carbonyl impurities affect the commercial quality and acceptability of the product acetic acid. If the concentration of carbonyl impurities reaches only 10-15 ppm, the commercial value of the product acetic acid will certainly be negatively affected. As used herein the phrase "carbonyl" is intended to mean compounds which contain aldehyde or ketone functional groups which compounds may or may not possess unsaturation.
It is postulated in an article by Watson, The Cativa.TM. Process for the Production of Acetic Acid, Chem. Ind. (Dekker) (1998) 75 Catalysis of Organic Reactions, pp. 369-380, that enhanced rhodium catalyzed systems have increased standing levels of rhodium-acyl species which will form free acetaldehydes at a higher rate. The higher rate of acetaldehyde formation can lead to the increased production of permanganate reducing compounds.
The precise chemical pathway within the methanol carbonylation process that leads to the production of crotonaldehyde, 2-ethyl crotonaldehyde and other permanganate reducing compounds is not well understood. One prominent theory for the formation of the crotonaldehyde and 2-ethyl crotonaldehyde impurities in the methanol carbonylation process is that they result from aldol and cross-aldol condensation reactions starting with acetaldehyde. Because theoretically these impurities begin with acetaldehyde, many previously proposed methods of controlling carbonyl impurities have been directed towards removing acetaldehyde and acetaldehyde derived carbonyl impurities from the reaction system.
Conventional techniques used to remove acetaldehyde and carbonyl impurities have included treatment of acetic acid with oxidizers, ozone, water, methanol, amines, and the like. In addition, each of these techniques may or may not be combined with the distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the product acetic acid. Likewise, it is known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxyl amine which reacts with the carbonyl compounds to form oximes followed by distillation to separate the purified organic product from the oxime reaction products. However, this method of treating the product acetic acid adds significant cost to the process.
There is disclosed in U.S. Pat. No. 5,625,095 to Miura et al. and PCT International Application No. PCT/US97/18711, Publication No. WO 98/17619 various methods of removing acetaldehydes and other impurities from a rhodium-catalyzed acetic acid production process. Generally, these methods involve extracting undesirable impurities from process streams to reduce acetaldehyde concentrations in the system.
The approaches described above have achieved a certain level of success in controlling carbonyl impurity concentrations in acetic acid produced by methanol carbonylation. Nonetheless, even with the use of these prior art removal methods, acetaldehyde and carbonyl impurities that derive from acetaldehyde, particularly, crotonaldehyde and 2-ethyl crotonaldehyde, continue to be a problem in product acetic acid produced by methanol carbonylation. Accordingly, a need remains for a method to control carbonyl impurities in product acetic acid produced by methanol carbonylation, particularly one which can be performed economically without adding to the impurities in the acetic acid or incorporating costly additional processing steps. It has been found that reduced levels of methyl iodide lead to improved purity profiles.